The psoas major is largest muscle in cross section at lower levels of the lumbar spine. It hasattavhment to all lumbar spine transverse processes and to the antero medial aspect of all lumbar transverse processes and to the anteromedial aspect of the lumbar discs and adjoining bodies with exception of L5 and S1 discs.

These attachment constitute the individual fascicles. These fascicles are oriented anferolaterally and come together as a common tendon which descends over the pelvic brim and shares common ibnsertion with iliacus muscle on the lesser trochanter of the femur.

Fascial relation of the psoas major :

Fascial relation of psoas needs special attention as biomechanics of connected structures is greatly affected. The medial arcuate ligament is a continuation of superior psoas fascia that continues superiorly to the diaphragm. It also attaches to the anterior longitudinal ligaments, pelvic floor fascia, transverse abdominus and internal oblique.

Biomechanics and function :

Function of psoas major is another area of controversy and uncertainty in literature. The well established action of psoas is hip flexion with illiacus.

Several hypothesis have been made til now.
Keagy et al. Performed EMG on five patients and found that psoas plays significant role in advancing and the limb while walking and deviation of trunk when sitting.

Natchemson and few other researchers found that psoas may play role with lumbar spine stability and movement. It plays as a flexor of lumbar over pelvis, a stabilizer of lumbar spine, stabilizer of the hip, power source for bipedal walking and running and maintaining lumbar lordosis when supporting difficult lumbar loads.

Yoshio et al used cadavers and concluded that psoas major works As an erector of the lumbar vertebral column, as well as a stabilizer of the femoral head into acetabulum( from 0 to 45). They also concluded that action of psoas as a hip stabilizer is over shadowed by its action as a lumbar stabilizer or erector.

Dangeria and Naesh found that most patients with lumbar disc herniation there was significant reduction in cross sectional are of the psoas major on the affected side only and most prominently at the level of disc herniation.

Barker et al. Investigated the cross sectional area of psoas major in unilateral low back pain through MRI and found that there were significant differences in cross sectional area of psoas major between sides and that there was a positive correlation between a decreased cross sectional area of psoas and duration of symptoms.

Hides et al found that with prolonged bed rest the cross sectional area of recus and psoas increased surprisingly. The reason may be, person keeps hip and leg flexed during rest which shortens the muscles.
Psoas major has intimate anatomical attachments to the diaphragm and the pelvic floor. This unique position allows psoas as a link between these two structure and may help in maintaining stability of lumbar cylinder mechanism.

Juker et al. Showed that the psoas major counteracts the action of iliacus during hip flexion. They believe that the illiacus would tourque the pelvis into anterior pelvic tilt and that the psoas major works against these forces, adding to the stiffness within the pelvis and lumar spine. An activated and stiffened psoas major will contribute some shear stiffness to the lumbar motion segment.

Take home message :

  • The Posas major(PM)on upper and lower lumbar  spine (LS) regions, performed passively and requiring minimal muscular effort, may serve to stabilize the LS in an upright stance.
  • PM probably functions as a stabilizer of the lordotic LS in an upright stance by adapting the state of contraction .
  • Always look for psoas flexibility and strength testing when gait deficits, low back pain .

Reference:

  1. Psoas Major: a case report and review of its anatomy, biomechanics, and clinical implications- Sandy Sajko, BPHE, DC, MSc and Kent Stuber, BSc, DC, MSc
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  6.  Jemmett RS, MacDonald DA, Agur AMR. Anatomical relationship between selected segmental muscles of the lumbar spine in the context of multi-planar segmental motion: a preliminary investigation. Man Ther. 2004;9:203–210.
  7. Basmajian JV. Electromyography of the iliopsoas. Anatom Record. 1958;132(2):127–132.
  8. Juker D, McGill SM, Kropf P, Steffen T. Quantitative intramuscular myoelectric activity of lumbar portions of psoas and the abdominal wall during a wide variety of tasks. Med Sci Sport Exercise. 1998;30(2):301–310.
  9. Penning L. Psoas muscle and lumbar spine stability: a concept uniting existing controversies. Eur Spine J. 2000;9:577–585.